Start Date: January 2005
Christopher E. D. Chidsey, T. Daniel P. Stack, Robert M. Waymouth, Department of Chemistry, Stanford University
This project aims to develop efficient catalysts for direct-hydrocarbon fuel cells. It is envisioned that these catalysts will be transition metal complexes mounted on carbon electrodes. Specifically, various configurations of late-metal multi-metallic catalyst complexes will be investigated for their role as electrooxidation catalysts, and biologically inspired mono- and multi-metallic copper complexes will be examined as electroreduction catalysts.
The complete oxidation of long-chain alkanes by dioxygen is described by the following approximate thermochemical equation:
In a fuel cell, six electrons must be transferred from anode to cathode for each CH2 group oxidized. Thus the work available is up to 105 kJ per mole of electrons or 1.09eV per electron, i.e., a reversible cell voltage of 1.09V. However, even with the recent progress made in direct-methanol fuel cells, operating voltages at reasonable current densities are still far below this reversible ideal. The dominant approach to electrocatalysis to date has been empirical and oriented to traditional heterogeneous catalysts comprised of solid-state materials dispersed within the porous electrodes. Such work, while likely to show the quickest incremental advances, is unlikely to lead to the kind of deeper understanding and precise tailoring of the required reactivity that modern chemistry is now capable of using homogeneous molecular catalysts.
The intent of this project is to investigate the potential of discrete molecular catalysts for hydrocarbon electrooxidation based on homo- and hetero-bimetallic palladium, platinum and ruthenium complexes and for O2 electroreduction based on copper complexes with heterocyclic aromatic ligands. A common theme of the approach is to investigate planar, acid-resistant ligand systems that will bind avidly to graphitic electrodes to facilitate rapid electron injection or removal from catalytic intermediates.
Hydrocarbon electrooxidation catalysts: Several recent studies have indicated that Rh and Pd catalysts are capable of both C-H and C-C cleavage reactions in the presence of O2. (Lynn et al., 1997) The mechanisms of these reactions have not been fully elucidated, but provide an important starting place for investigating the role of ligand environment and electrochemical potential on the selective and energy-efficient activation of hydrocarbons on electrode surfaces. The decarbonylation of metal acyl species or the decarboxylation of carboxylic acids provides one set of strategies to the oxidative degradation of C-C bonds (Henry, 1980), the oxidative addition of ketones provides another strategy (Jun et al., 1999, and Lee et al., 2003), and the dehydrogenation of carboxylic acids, followed by oxidation and retro-Claisen condensation (Hamed et al., 2001) provides a biomimetic strategy for C-C cleavage (see Figure 1).
In order to facilitate the activation of C-H bonds and the cleavage of C-C bonds in higher hydrocarbons, this project will also investigate the catalytic oxidation of CO to CO2. CO oxidation has been proposed as one source of the required large overpotentials in direct hydrocarbon fuel cells. A specific objective of this program is to investigate the possible cooperative role of the Ru center in facilitating several key steps in the oxidation of methanol to carbon dioxide. There may be several roles of the bridging Ru-OH group: (1) as a nucleophile to assist in the oxidation of Pt/Pt CO complexes to CO2, and (2) as a potential proton shuttle to deprotonate cationic Pd or Pt hydrides.
Oxygen electroreduction catalysts: While many metals could be investigated as a low overpotential, O2-electrocatalyst, copper is attractive, since fungal laccase enzymes dispose of low-energy electrons by reduction of O2 at a trinuclear copper active site at very positive potentials. Fungal laccases are a subset of the multi-copper oxidases that are found in most life forms (Solomon, et al., 1996). A second protein that provides key insights into the efficient reduction of O2 by copper is hemocyanin, the O2-transport protein of arthropods and mollusks (Solomon, et al., 1996). These proteins reversibly bind O2 through a side-on peroxide-level intermediate (see Figure 2). As this process is reversible at ambient temperatures, the initial 2e- reduction of O2 is energetically efficient providing keen insights into the initial steps of an efficient 4e- reduction of O2.
For the oxygen electrode, this project will use simple, oxidatively robust copper assemblies adsorbed directly onto a graphite electrode. The ligation of the copper(s) and their organization will parallel that observed in the enzymes that reversibly interact with O2 or reduce O2 with low overpotentials (Mirica et al., 2004). Site-isolation and strongly coordinating ligands for copper will be necessary in these simple complexes to create a functioning O2-electrocatalyst capable of operating under the acidic conditions of an efficient fuel cell.
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